An unknown investor tried to buy a 20 per cent stake in Chelsea shortly before the Premiership football club announced it was being acquired by Russian billionaire Roman Abramovich.

The attempted purchase is to be investigated by the Financial Services Authority, which has launched an insider-dealing probe into the way Chelsea's share price surged just before the Abramovich deal was announced on 2 July.

A few days before that announcement, Ruth Gist, widow of the late Chelsea tycoon Matthew Harding, received an offer for her 21 per cent shareholding in Chelsea Village, the football club's parent company. The offer, which was turned down, valued her stake at the then-market price of 20 to 25p a share. Abramovich's agreed takeover has since boosted Chelsea's shares to 35.5p.

Richard Parry, Gist's solicitor, confirmed yesterday that the approach had been made but declined to comment further. However, Gist is 'relieved' that she did not sell, because the value of her stake has since risen by some £4 million.

Parry refused to identify the would-be buyer, citing commercial confidentiality. He is thought to have no known previous connection to Chelsea.

A spokesman for the FSA declined to comment yesterday. According to insiders, however, the City regulator is keen to make sure that the mystery speculator had no prior knowledge of Abra-movich's intentions. Before the Russian stepped in, Chelsea was saddled with £90m of debts and facing a cash crunch.

Separately, the FSA disclosed last week that it is concerned the market may have been 'misled' over the true ownership of certain minority shareholdings in Chelsea. But it does not expect this new strand in its investigations to threaten the Abramovich deal.